Reset `Startables` COUNTER when testing parallel startup

Closes gh-43369

spring-boot-project/spring-boot-testcontainers/build.gradle

@@ -96,3 +96,7 @@ dependencies {
 	testImplementation("org.springframework.pulsar:spring-pulsar")
 	testImplementation("org.testcontainers:junit-jupiter")
 }
+
+dockerTest {
+	jvmArgs += "--add-opens=java.base/java.util.concurrent=ALL-UNNAMED"
+}

spring-boot-project/spring-boot-testcontainers/src/dockerTest/java/org/springframework/boot/testcontainers/lifecycle/ResetStartablesExtension.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2012-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.testcontainers.lifecycle;
+
+import java.lang.reflect.InaccessibleObjectException;
+import java.util.concurrent.atomic.AtomicLong;
+
+import org.junit.jupiter.api.extension.AfterEachCallback;
+import org.junit.jupiter.api.extension.BeforeEachCallback;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.testcontainers.lifecycle.Startables;
+
+import org.springframework.test.util.ReflectionTestUtils;
+
+/**
+ * JUnit extension used by reset startables.
+ *
+ * @author Phillip Webb
+ */
+class ResetStartablesExtension implements BeforeEachCallback, AfterEachCallback {
+
+	@Override
+	public void afterEach(ExtensionContext context) throws Exception {
+		reset();
+	}
+
+	@Override
+	public void beforeEach(ExtensionContext context) throws Exception {
+		reset();
+	}
+
+	private void reset() {
+		try {
+			Object executor = ReflectionTestUtils.getField(Startables.class, "EXECUTOR");
+			Object threadFactory = ReflectionTestUtils.getField(executor, "threadFactory");
+			AtomicLong counter = (AtomicLong) ReflectionTestUtils.getField(threadFactory, "COUNTER");
+			counter.set(0);
+		}
+		catch (InaccessibleObjectException ex) {
+			throw new IllegalStateException(
+					"Unable to reset field. Please run with '--add-opens=java.base/java.util.concurrent=ALL-UNNAMED'",
+					ex);
+		}
+	}
+
+}

spring-boot-project/spring-boot-testcontainers/src/dockerTest/java/org/springframework/boot/testcontainers/lifecycle/TestContainersParallelStartupWithImportTestcontainersIntegrationTests.java

@@ -40,7 +40,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 @ExtendWith(SpringExtension.class)
 @TestPropertySource(properties = "spring.testcontainers.beans.startup=parallel")
 @DisabledIfDockerUnavailable
-@ExtendWith(OutputCaptureExtension.class)
+@ExtendWith({ OutputCaptureExtension.class, ResetStartablesExtension.class })
 @ImportTestcontainers(Containers.class)
 class TestContainersParallelStartupWithImportTestcontainersIntegrationTests {